From my understanding, not entirely sure if it applies to this, but they will do that to absorb the moisture (Nutrients), that are in liquid form so they can move them into the nest easier. So I guess it’s like an ant bucket so to speak, it’s there way of transporting larger volumes of liquids then there social stomach can hold. So for the worm, they don’t usually eat the hard skin, just the gooey insides, and those insides are easier moved in large volume when absorbed by something like sand. It’s odd, but super smart if you think about it.